Cisplatin is highly effective in the treatment of various cancers. Resistance to this drug, however, poses a major problem to the success of chemotherapy. Several possible mechanisms that can contribute to platinum resistance have been identified; however, a clear correlation between the known in vitro mechanisms of cisplatin resistance and clinical outcome is lacking. This is likely a result of alterations in multiple genes within tumors. To improve the efficacy of cisplatin treatment, it may be necessary to use multiple tractable molecular markers for prediction of tumor response to chemotherapy. The long-term goal of this project is to identify novel genes and/or molecular pathways, which mediate cisplatin resistance. The approach has been first to identify yeast genes, which when deleted, confer resistance to cisplatin and then to evaluate the human homologues for their role in cisplatin toxicity. By screening the collection of S. cerevisiae deletion mutants created by the Yeast Genome Deletion Project, several novel yeast genes, which when deleted confer resistance to cisplatin, have been identified. Genes involved in two cellular pathways were chosen for further study because they were identified multiple times and/or when deleted conferred higher resistance, and they have human homologues. First are the FCY2 and HPT1 genes involved in the AMP biogenesis pathway; second are 2 genes in the NMD (nonsense-mediated mRNA decay) family, which are involved in mRNA surveillance and regulation of translation. The specific aims of this application are: 1) to characterize selected biochemical mechanisms by which the identified genes may mediate cisplatin resistance in yeast, and 2) to evaluate the human homologues for their roles in cisplatin resistance using a genetic complementation assay and the RNAi interference strategy. Identification of new mechanisms accounting for cisplatin resistance is of fundamental importance. Evaluation of these targets may identify new biomarkers for pre-treatment screening of tumor samples for mutations that could allow for the identification of patients who are unlikely to benefit from cisplatin treatment.